Treatment of silicon-iron alloys



Patented May 5, 1942 TREATMENT OF SILICON-IRON ALLOYS Stephen L.Burgwin, Wilkinsburg, Pa., assignmto Westinghouse Electric &Manufacturing. Company, East Pittsburgh, Pa., a corporation oiPennsylvania No Drawing.

6 Claims.

Application-February. 18, 1941, Serial No. 379,481

employed for heat treating the silicon-iron al- J loys and fabricatingthem into strip form suitable ior use as the magnetic cores ofelectrical apparatus, The known processes and methods have for theirobjective the development of the magnetic characteristicsof the alloy byreducing the impurities in the alloy, relieving working strains and/orimparting a preferred orientation to the grain structure of the alloy.The silicon-iron alloys now commercially produced in strip form haveexcellent magnetic characteristics.

In a number of commercial installations of electrical apparatus, such'aslarge power transformers which utilize the now commercially producedsilicon-iron alloy in strip form as the magnetic core member, it hasbeen found that undesirable and objectionable noise is developed whenthe transformer is energized. The noise developed in a transformer canbe traced directly to the magnetostriction oi the silicon-iron alloystrips utilized as the core member of the transformer. Themagnetostriction of magnetic material is recognized as being theexpansion and contraction of the magnetic material when subjected to amagnetizing force. For the purpose of this invention, the kind ofmagnetostriction of particular interest and the quantity measured is orthe change in length per unit length in the direction of magnetizationunder the magnetizing force applied.

An object of this invention is to provide for reducing themagnetostriction or silicon-iron alloys which have been fabricated intostrip form and treated to develop the magnetic characteristics of thealloy.

A more specific object of this invention is to subject silicon-ironalloys which have been fabricated into strip form and treated to developthe magnetic characteristics of the alloys to a heat treatment suitablefor placing the alloys under tension to effectively reduce themagnetostriction of the alloy strip.

Other objects of this invention will become veloping the magneticcharacteristics of such alloys have been commercially practiced in thepast. For example, silicon-iron alloys containing up to about l ofsilicon are commercially produced in strip form by subjecting them todifferent heat treatments including the step of cold rolling them tothermal stages and then subjecting the resulting strip to an anneal at atemperature'of 900 C. or higher to develop the magnetic characteristicsof the alloy. Silicon-iron alloys containing higher contents of siliconup to about 6% are not being commercially produced by cold rollingprocesses, since the higher silicon content renders them somewhatbrittle and it has, therefore, become cuss tomary to hot work suchalloys to the final stages after which a final anneal is employed forrelieving working strains.- Other methods are well-known and practicedfor developing the magneticcharacteristics of the silicon-iron alloyscontaining up to 6% of silicon.

In order to reduce the magnetostriction oi the commercially producedsilicon-iron alloys containing up to 6% of silicon, in accordance withthis invention the commercially produced alloy strips are subjected'to afurther heat treatment for permanently placing the alloyed components ofthe strip under tension to effectively reduce the magnetostrictionof thecommercially produced alloys. The heat treatment of this inventionconsists in heating the commercially produced alloy strip at a preferredtemperaturev between 700 C. and 1200" C. in the presence of a componentwhich is capable of reacting with the silicon-iron alloy to form a thinadherent film on the surfaces of the strip, the thin film having alinear thermal coefflcient of expansion less than the linear thermalcoeflicient of expansion oi the silicon-iron alloys so that in thecooledstate the adhering film is present in compression while thesilicon-iron alloy is maintained under tension. Many reagents may beemployed in forming the adhering film on the silicon-iron alloy strip,it being essential, however, that the reagent be selected to produce aresulting film having a linear thermal coeflicient of expansion of lessthan the coeiiicient of expansion of the silicon-iron strip on which itis formed.

apparent from the following description and the In a preferred methodembodying the teachings of this invention, the treated alloy strip issubjected to a heat treatment at a temperature between 700 C. and 1200C. in an oxidizing atmosphere, such as air, moist air or carbon dioxideatmosphere or a combination thereof for a period' of time of not morethan 8 minutes, the oxidizing atmosphere reacting with the alloy to forma thin film of oxides which adhere to the alloy strip. The oxide filmusually contains silicates. Any suitable furnace may be employed forheating the strips to the desired temperature as they are treatment atless than that time which will cause fiaking of the oxide when the stripis cooled.

In a modification of the method described hereinbeiore, tension'isapplied to the strip dur ing theheat treatment for the purpose of insuring that the alloying components will be so; strained that as. the stripis cooled, the film of oxides formed on the strip during the heat treat;ment will retain th alloying components under tension even aftertheexternal tension is removed. In applying tension during the heattratment, it is desired, howevento maintain the.

The heat treatment given each of the samples in an atmosphere of air isindicated in the above table. In all cases thereis a definiteimprovement in the magnetostriction values obtained after thecommercially produced strip has'been treated in accordance with thisinvention. From the results given, it is evident that a preferredtreatment is to oxidize the commercially produced strips at atemperature between 925 C. and 1100C. vfor a length of time rangingbetween one and four minutes.

The films formed by the oxidizing treatment are of the order of 2 10-inches thick. The

* films formed have a definite effect inplacing the tension belowtheelastic limit of the. alloy strip, a

' it being foundthat tensions of the order of 500 to 2 000 lbs. persquare inch during the heating cycle is s ufficient for cooperating withthe oxide film for effectively reducing the .magnetostriction of thealloy strip.

It is not known exactly what change takes place inthe components orstructure ofthe alloy when the adhering film is formed thereon and thestrip is cooled. The results obtained, however, illustrate quitedefinitely that the combination of the thin film under compression withthe silicon-iron alloy under tension effectively reduc es themagnetostriction of the commercially produced silicon-iron strip, l i Indetermining the effect of ,the thin films formed on the commerciallyproduced strips, dif:

ferent methods have been :developedfor measur- I ing the,magnetostrict-ion'of the alloy strips." The results obtained by thedifferent methods are quite close and for this reason,-it is believed tobe unnecessary to describe the different methods of measuringthe-magnetostriction of the alloy strip when treated in accordance withthis in-. vention. Two methods for measuring the mag;

netostriction are, however, described in my 00- pending application,Serial No; 385,759, filed March 28, 1941, the methods described thereirfbeing suitable for measuring the magnetostriction of the strips treatedby the method of this inven-l tiOIl'. i

In order to illustrate the improvement in the: magnetostriction ofcommercially produced strips when subjected to the treatment describ'edthe followingtable may be referred to as givingfthe;

magnetostriction values,-that is, change in length; per length at agiven magnetization of l silicon-iron alloy strips. after the commercialmill anneal and after it has been given'the oxidizing treatment forforming the thin adhering} films thereon in accordance with thisinvention.

silicon-iron alloy under tension and the stresses 7 the other. This hasbeen accomplished by electrolytic etching the filmirom the strip with a10% solution of sulphuric acid, measuring the bending-of :the strip "andthen calculating the tensile stress'based on the bending of the strip;

Tensions of the order of 1200 lbs. per square inch have been found instripstreated in accordance with this invention. v

In another embodiment of this invention, sodium silicate was employedforapplying a film during the heat treatment for effectively reducing themagnetostriction "of the commercially produced alloys; In employing thesodium silicate, the commercially produced alloy strip is coated with athin film of sodium silicate solu- .tion and-then'dr-ied at about 200C., after which the coated strip issubjected to the heat treatment at atemperature between 875 C. and 1000.C.'in a'ir fora period of time ofabout two minutes. When cooled,'it is iound that a very adherent film isformed on the treated strip and when the magnetostriction of the stripis measured, it is found that a definite improvement in themagnetostriction is obtained. In the following table, there is given theaverage magnetostriction values obtained on a number of commerciallyproduced l silicon-iron alloy strips before and after thesodium-silicate treatment described.

Avcrngcmagnctosi rim ion 5X10 -minutes in air.

'l'cst number B'=l0000 B=l-l000 Mill Mill anneal Tl'LfitLd annealTreated Each of tests58, 59 and 60 is the average of a number of testson strips of the same composition which were subjected to the heattreatment at 900 C. for two minutes in air while strips upon which theresults for test 61 are based were of a carbonate, sulphate or'silicatesolution. The

coating formed of the solution will evolve oxygen during the heattreatment to effect an adjustment in the oxygen content of the annealingatmosphere and thereby control the formation of the adhering oxide filmon the alloy strip. Strontium carbonate can be effectively employed inthis manner for controlling the formation of the adhering oxide film onthe strip. Further, by

'employing the superficial films formed from the solutions ofcarbonates, sulphates or silicates, it is possible to utilize hydrogenor nitrogen or combinations thereof as the annealing atmosphere, theoxygen evolved from the superficial films being sufficient to effect theformation of the adhering oxide films to establish stresses in tensionin the alloystrip and reduce the magnetostriction of the strip. v

From the results given hereinbefore, it is quite evident that the methodof treating the siliconiron alloy containing up to 6% of silicon whichhas been fabricated into strip form and treated to develop its magneticcharacteristics at a temperature between 700 C. and 1200 C. in thepresence of a component capable of reacting with the silicon-iron alloyto form a thin film on 5 the surface of the strip for placing the alloyunder tension effectively reduces the magnetostriction of the alloy.

While this invention has been described with reference to particularembodiments thereof, it is, of course, not to be limited thereto exceptinsofar as is necessitated by the prior art and the scope of theappended claims.

I claim as my invention:

1. In the method of reducing magnetostrict-ion in silicon-iron alloycontaining up to 6% of silicon and which has been fabricated into stripform and treated to develop its magnetic characteristics, incombination, coating the treated alloy strip with sodium silicate,subjecting the coated strip to a heat treatment at a temperature between900 C. and 1l00 C. in air for a period of time of not more than threeminutes to effectively oxidize the strip and form a thin fused adheringfilm thereon, and cooling'the alloy strip with the adhering film ofoxidized and fused material thereon, the adhering film having a linearthermal coefficient of expansion lessthan the linear thermal coefficientof expansion of the silicon-iron alloy to give a strip in which thesilicon-iron alloy is under tension and the adhering film is incompression to effectively reduce the magnetostriction of the alloystrip.

2. In the method of reducing magnetostriction in silicon-iron alloycontaining up to 6% of silicon and which has been fabricated into stripform and treated to develop its magnetic characteristics, incombination, heating the treated alloy strip to a temperature between700 C. and 1100 C. in the presence of a reagent capable of m reactingwith the silicon-iron alloy to form a thin film of oxidized material onthe surfaces of the strip while applying tension to the strip, thetension applied being below the elastic limit of the alloy strip, andcooling the alloy strip ,5 with the thin film of oxidized materialthereon as an adherent part thereof, the reagent being selected to forma thin film of oxidized material having a linear' thermal coefficient ofexpansion less than the linear thermal coefficient of :0 expansion ofthe silicon-iron alloy to give a strip in which the silicon-iron alloyis under tension and the adhering film of oxidized material is incompression to effectively reduce the magnetostriction of the alloystrip.

3. In the method of reducing magnetostriction in silicon-iron alloycontaining up to 6% of silicon and which has been fabricated into stripform and treated to develop its magnetic characteristics, incombination, heating the treated allo strip at a temperature between 925C. and 1100 C. in an oxidizing atmospherefor a period of time ofnot-more than five minutes to form a thin film of oxidized material onthe surfaces of the strip while applying tension to the strip, thetension applied being below the'elastic limit of the alloy strip, andcooling the oxidized strip to retain the oxidized film as an adherentpart thereof, the thin oxidized film having a linear coefficient ofexpansion less than the linear coefficient of expansion of thesilicon-iron alloy to give a strip in which the adhering oxidized filmmaintains the silicon-iron alloy under tension to effectively reduce themagnetostriction of the alloy strip.

4. In the method of reducing magnetostriction in silicon-iron alloycontaining up to 6% of silicon, and which has been fabricated into stripform and treated to develop its magnetic characteristics, incombination, heating the treated alloy strip to a temperature between825 C. and 1100 C, in the presence of a reagent capable of reacting withthe silicon-iron alloy to form a film of oxidized material containingsilicates having a thickness of about 2 l0- inches thick on the surfacesof the strip while applying tension to the strip. the tension appliedbeing below the elastic limit of the alloy strip, and cooling the alloystrip with the film of oxidized material containing silicates thereon asan adherent part thereof, the film of oxidized material containing thesilicates having a linear thermal coefficient of expansion less than thelinear coefficient of expansion of the silicon iron alloy to give astrip in which the silicon-iron alloy is under tension and the adheringfilm of oxidized material containing silicates is in compression toeffectively reduce the magnetostriction of the alloy strip.

5. In the method of reducing magnetostriction in silicon-iron alloycontaining up to 6% of silicon and which has been fabricated into stripform and treated to develop its magnetic characteristics, incombination, heating the treated alloy strip to a temperature between825 C. and 1100 C. in the presence of a reagent capable of reacting withthe silicon-iron alloy to form a thin film of oxidized material on thesurfaces of the strip while applying tension to the strip, the tensionapplied being between 500 and 2000 pounds per square inch and maintainedbelow the elastic limit of the alloy strip, and cooling the alloy stripwith the film of oxidized material thereon as an adherent part thereof,the thin film of oxidized material having a linear coefficient ofexpansion less than the linear coefficient of expansion of thesilicon-iron alloy, the tension applied during the heating so effectingthe straining of the aloying components of the silicon-iron strip thatthe adherent film of oxidized material having the lower coefficient ofexpansion maintains the silicon-iron alloy under tension when the stripis cooled to effectively reduce the magnetostriction of the alloy strip.

6. In the method of reducing magnetostrictlon in silicon-iron alloycontaining up to 6% of silicon and which has been fabricated into stripform and treated to develop its magneticcharacteristics, in combination,coating the treated alloy strip with sodium silicate, subjecting thecoated strip to a heat treatment at a temperature between 900 C. and1100" C. in air ior a period of time of not more than three'minutestoeffectively oxidize the strip andform a thin fused adhering film thereonwhile applying tension to the strip, the tension applied being between500 :and 2000 pounds per square inch and maintained below the elasticlimit of the strip, and cooling the alloystrip with the adhering film ofoxidized which the silicon-iron alloy is under tension and theadheringfilm is in-compression to eflectively reduce the magnetostriction of thealloy strip.

STEPHEN L. BURGWIN.

